No motive has been given, but authorities say the area has had a history of land disputes.

The region has in the past also seen tensions between minority ethnic Mongolians and Han Chinese.

Police did not say how many people had been detained.

'Forklifts, sticks and pepper spray'

Some 13 people were injured in the early morning attack in Ejin Banner county, which lasted about two hours, reports said. Two of them were checkpoint staff, while the rest were "herdsmen and farmers defending the frontier", said a Global Times report.

The attackers were armed with sticks and pepper spray. The victims were reportedly beaten, robbed of their valuables, and then tied up and left outside in freezing -20C weather.

Two forklifts were then used to smash into checkpoint buildings as well as several vehicles, local media said.

Local official Li Yanbo told reporters that the area has long been the source of dispute among locals due to the repeated redrawing of provincial boundaries in the 1960s and 1970s.

The Global Times said there had been several clashes between Inner Mongolians and residents of neighbouring Gansu province in the area.

In September, the same outpost had been visited by masked men carrying sticks who intimidated staff. The men later drove away after staff persuaded them to leave.